At the 2010 Winter Olympics Košir reached sixth place in the parallel giant slalom. Four years later at the 2014 Winter Olympics , he won both a silver medal in the parallel slalom and a bronze medal in the parallel giant slalom.
The following season 2014/2015 was his most successful in the Snowboard World Cup . In the parallel slalom and the parallel giant slalom, he was on the podium in seven out of nine races. He was able to secure the overall parallel World Cup victory that year.